Service Providers behaviour for dual homed enterprises

Thu Sep 24 14:27:48 UTC 2015

Thank you all for answering.  I was disregarding Local Pref because the
route server I was on was showing 100.  That was an error on my part though
as it clearly states in the login banner that it is eBGP peering with the
AT&T routers hence the local Pref would go back to 100 from its
perspective.  Again, thanks for the quick and thorough responses.

On Thu, Sep 24, 2015 at 10:05 AM, Blake Hudson <blake at ispn.net> wrote:

>
>
> Stephen Satchell wrote on 9/24/2015 8:39 AM:
>
>> On 09/23/2015 02:38 PM, Jason Bullen wrote:
>>
>>> I've always worked in enterprise only so I thought you guys might be able
>>> to help me with this one.
>>> We are dual homed to Verizon and AT&T.  We prepend all our prefixes out
>>> AT&T to make them least preferred.  During a recent issue we found some
>>> users were coming in via AT&T.  Using various looking glasses it looks
>>> like
>>> if I use an AT&T server(route-server.ip.att.net) the best path is the
>>> prepended route through AT&T; in fact,I don't even see the VZB route.
>>> If I
>>> use a 3rd party looking glass(router-server.he.net) I see what I
>>> anticipated, which is the shorter AS-Path through VZB.
>>>
>>> So if my research is correct, the internet prefers Verizon UNLESS they
>>> are
>>> a direct AT&T customer then they would use the AT&T circuit.
>>> Is this a standard practice that I should assume to encounter?
>>>
>>> Thanks in advance
>>>
>>>
>> That's been my experience, and with other sets of providers, too.
>>
>> My current company is dual-homed with AT&T and Charter Fiber. Those
>> customers on UVerse come in the AT&T link no matter what we do with BGP to
>> convince the cloud to let packets come in the fatter pipe.
>>
>
> Jason, while others have offered acknowledgement of the behavior you are
> seeing as well as solutions, I think it might be relevant to point out that
> this is simply a matter of BGP best path selection. BGP does not use AS
> path length (hops) as its primary path selector. Search for "bgp best path
> selection" to find out more about how BGP selects the best path. As others
> have noted, local pref is often utilized to control routing and should be
> your preferred way to control path selection in addition to AS path length.
> However, the ultimate way to control routing would be to advertise more
> specific prefixes via the path that you want traffic to flow.
>
> --Blake
>